Created attachment 120376 [details]
archive containing program, patch and screenshots of the issue

There is an inconsistency in line visibility for lines of zero length
depending on the line-width attribute. This happens when using SNA instead of UXA with xf86-video-intel driver.

I have done a program to show this issue (see test.c and screenshots in attachment). The program draw a grid of sample lines with varying parameters:
- following X axis: increasing line length (0 to 2)
- following Y axis: increasing line-width (1 to 3)

Normally we expect to have the same behavior on each row of the grid (either visible lines or not) but in the case of zero length lines, we obtain a visible line only if line-width == 1.

From the docs [1] it is said that for a line-width >= 1, zero length lines should not be visible when drawn with CapButt attribute (as in our case).

I have put a patch in attachment solving the issue (patch.diff). Bug and patch have been tested with Debian Jessie official version (2.99.917) and also with git latest commit (da9ad388).

[1] http://tinf2.vub.ac.be/~dvermeir/manual/xlib/GC/manipulating.html

environment
-----------

uname -a:
Linux crunchbang 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1 (2015-05-24) x86_64 GNU/Linux

lspci -nn:
00:02.0 VGA compatible controller [0300]: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller [8086:0416] (rev 06)

versions:
- xorg: Debian Jessie official releases
- xf86-video-intel: bug present in either Debian Jessie release (2.99.917) or latest git commit (da9ad388)

The driver used is i915 with SNA.

I can provide other logs if needed.